An image displayed on a cathode ray tube (CRT) may suffer from imperfections or distortions such as defocusing or nonlinearity that is incident to the scanning of the beam on the CRT. Such imperfections or distortions occur because the distance from the electron gun of the CRT to the faceplate varies markedly as the beam is deflected, for example, in the horizontal direction. Reducing the defocusing that occurs as the beam is deflected in the horizontal direction, for example, may be obtained by developing a dynamic focus voltage having a parabolic voltage component at the horizontal rate and applying the dynamic focus voltage to a focus electrode of the CRT for dynamically varying the focus voltage. It is known to derive the parabolic voltage component at the horizontal rate from an S-correction voltage developed in an S-shaping capacitor of a horizontal deflection output stage.
The CRT that employs dynamic focus may have internal wiring that places the dynamic focus voltage close to, for example, the blue electron gun. In normal operation, the proximity to the blue electron gun may not cause any problem. However, when a low current bias measurement is made in an automatic kine bias (AKB) circuit, during several video line times that immediately follow vertical retrace, referred to as the AKB measurement interval, stray coupling of the horizontal component of the dynamic focus voltage may introduce an error in the biasing of the cathode electrode of the blue electron gun. As a result, the bias of the blue electron gun may not track the bias of the green and red electron guns. This may lead to unacceptable background color temperature changes.
It may be desirable to remove the horizontal dynamic focus voltage component from the focus electrode, during the AKB measurement interval. Thereby, the undesirable coupling to the focus electrode is, advantageously, eliminated. It may be desirable to start disabling the dynamic focus voltage, prior to the AKB measurement interval, for preventing the occurrence of a transient condition in the dynamic focus voltage, during the AKB measurement interval. Therefore, the switch begins decoupling the dynamic focus voltage from the focus electrode, for example, close to the beginning time of the vertical blanking interval.
In a video display monitor, the time available for vertical retrace is a small portion of the vertical cycle. The addition of the AKB measurement interval following the end of the vertical blanking interval, disadvantageously, reduces the time available for vertical retrace. It may be desirable to allocate as much time as possible for the retrace interval for reducing the stress of the vertical deflection amplifier power transistor. Therefore, it may be desirable to reduce the tolerances of the end time of the interval, during which the dynamic focus voltage is disabled. This is so because, if the interval, during which the dynamic focus voltage is disabled, were to extend into the active video display interval, a portion of the visible scan line in the overlap region, disadvantageously, might appear not focused. On the other hand, if the interval, during which the dynamic focus voltage is disabled, were to end too early, the crosstalk of the dynamic focus voltage might cause an AKB error on one or more electron guns.
In accordance with an aspect of the invention, the end time of the interval, during which the dynamic focus voltage is disabled, is established accurately using a line timer or counter. The line timer or counter counts, for example, horizontal retrace pulses and produces a switch control signal for controlling the aforementioned dynamic focus voltage disabling switch. Advantageously, counting pulses provides more accurate measurement of the end of the interval, during which the dynamic focus voltage is disabled, than if such measurement were entirely dependent on tolerances of components such as resistors and capacitor.
A video imaging apparatus, embodying an inventive feature includes an amplifier responsive to a focus voltage correction signal for generating at an output of the amplifier a dynamic focus voltage component of a focus voltage that is coupled to the focus electrode. A switch is coupled in a signal path of the focus voltage correction signal for disabling the dynamic focus voltage component, during an automatic kine bias measurement interval of a deflection cycle. A source of a first signal indicative of an end time of a vertical blanking interval in the deflection cycle is provided. A synchronous time shifter is responsive to a clock signal and to the first signal for time shifting the first signal. The time-shifted first signal is coupled to the switch to control when the dynamic focus voltage is enabled.